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Adjustments/Posterize is making orange steps on gray ramps in Lab color mode.

Explorer ,
Jun 06, 2024 Jun 06, 2024

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Photoshop version: 25.9.0
Mac OS 12.7.5
 
Issue: When I apply Image/adjusments/posterise to a gray ramp in a Lab color mode document, even values of posterize levels  colors the image steps orange instead of gray.
 
However, odd values of posterize levles color the image in gray steps as expected.
 

Lab Posterize bugLab Posterize bug

 

Steps
1: create new document in 16 bit lab color mode.
2: with the gradient tool make a "classic" gradient from black to white.
3: choose application menu 'image/Adjustments/Posterize'
4: Choose an odd number for 'Levels'
 
Expected result. : posterized image should be steps of gray
Actual result: posterized image steps are orange.

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Adobe
Explorer ,
Jun 06, 2024 Jun 06, 2024

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opps, step 4 should read...

4: Choose an EVEN number for levels.

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Community Expert ,
Jun 07, 2024 Jun 07, 2024

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Yes, happens here too (on Windows).

 

Something wonky here. Sometimes it's orange, sometimes it's green. Here's what I get with a gradient fill layer, and then a posterize adjustment layer over that. This is a standard black to white gradient:

 

gradient.png

Only in Lab. RGB is fine.

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Community Expert ,
Jun 07, 2024 Jun 07, 2024

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From a practical perspective, target/select the L channel before you run Posterize (or uncheck the A and B channels in advanced blending).

 

Lab mode is the only mode that truly separates colour from tone. Use that accordingly.

 

As the A and B channels contain mid-grey for a neutral image, they in *theory* shouldn't be affected by posterize even if they are selected. So much for theory.  :]

 

Compare the result of posterize on an alpha channel copy of the A or B channel.

 

It's most likely a bug, I'm guessing it's been there forever.

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Community Expert ,
Jun 07, 2024 Jun 07, 2024

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@Stephen_A_Marsh 

I'm guessing it's been there forever.

 

Yes, probably. Not something you would come across often. I could probably have a long and prosperous careeer without ever seeing this 😉

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Community Expert ,
Jun 07, 2024 Jun 07, 2024

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BTW it's everything in Lab, not just gradients. It does this to images too, whether monochrome or color.

 

My guess is that the algorithm "expects" full range a and b channels. We should probably just consider posterize an RGB tool, not applicable in Lab.

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Explorer ,
Jun 07, 2024 Jun 07, 2024

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@D Fosse We should probably just consider posterize an RGB tool, not applicable in Lab.

 

It works fine on the lab images I tried*. Looking just as I would expect, but only for odd number posterize levels.  It's even numbers posterize levels it gets wrong. So a fix should not be hard.

(* Roman16 test images in Lab color mode)

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Explorer ,
Jun 09, 2024 Jun 09, 2024

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LATEST

This report has been moved out of the photoshop "bugs" section and into "discussions".

I can only presume the list manager had some doubts about this behaviour actually being a bug.

 

Please could this report be moved back into the "bugs" section.
 

For list users reading this, if you see the same behaviour, would you please consider 'up-voting' this. And leave a comment that you don't consider this normal behaviour. Hopefully we can get this treated as a bug and fixed. 

 

Thanks list subscribers for your help.

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